From the 1910s until his death, Van Loon wrote many books, most notably The Story of Mankind, by far his most well-known book, a history of the world especially for children which won the first Newbery Medal in 1922. The book was later updated by Van Loon and has continued to be updated, first by his son and later by other historians.

However, he also wrote many other very popular books aimed at young adults. As a writer he was known for emphasizing crucial historical events and giving a complete picture of individual characters, as well as the role of the arts in history. He also had an informal style which, particularly in The Story of Mankind, included personal anecdotes.

Asked how to say his name, he told The Literary Digest “I still stick to the Dutch pronunciation of the double o—Loon like loan in ‘Loan and Trust Co.’ My sons will probably accept the American pronunciation. It really does not matter very much.” (Charles Earle Funk, What’s the Name, Please?, Funk & Wagnalls, 1936.)